An important component of rotary drilling operations in the exploration for and production of subterranean resources is the drilling mud. Some of the most serious problems encountered in producing and maintaining effective clay-based aqueous drilling fluids are caused by the interaction of the mud with the earth formation being drilled. These interactions include contamination of the mud by formation fluids, incorporation into the mud of viscosity producing and inert drilled solids, chemical contamination by drilled solids, and heating of the mud by high temperature formations. These interactions generally cause extreme thickening of the muds and associated, costly drilling problems and delays.
One significant type of contaminant is minerals containing divalent ions such as calcium and magnesium. These materials can cause uncontrolled thickening of the mud and large increases in filtration of fluids from the mud into permeable formations. These minerals can be sourced from: the formation solids cut by the drill bit; formation fluids; or, the cement placed intentionally in the borehole during casing or shut-off operations and cut by the drill bit when drilling is resumed.
The effect of high temperature on the mud is also especially significant. Not only does heat itself cause flocculation of the mud, but it also increases the thickening effects of chemical contaminants such as calcium chloride, and other species and deactivates or destroys many mud thinners used to stabilize muds to these effects. As drilling depth has increased, so has the bottom hole temperature. Therefore, a product is required which can protect the mud properties under the adverse conditions of today's hotter, deeper well bores.
Ammonium and alkali metal salts of polymers of acrylic acid having a weight average molecular weight (Mw) in excess of about 2,000 (generally desirably in the range of 5,000 to 50,000; preferably in the range of 15,000 to 35,000--see col. 5, line 69 through col. 6, line 5 of U.S. Pat. No. 2,552,775) have been taught for use as a protective colloid for water based drilling muds. Further, copolymers of acrylic acid, acrylamide, or mixtures thereof and alkali metal ethylene sulfonate have been taught as usefully employed as tanning agents, ion exchange resins, etc. and in oil well drilling muds (U.S. Pat. Nos. 3,706,717 and 3,203,938).
Over the past several years various materials have been disclosed as high temperature water based mud deflocculants. Among these are: a low molecular weight poly(acrylic acid) of Mw not more than about 2500, and salts thereof (see U.S. Pat. No. 3,764,530--col. 7, lines 27-28); an acrylic acid homopolymer or salt thereof, said homopolymer having a weight average molecular weight of from about 2,500 to 5,000 which is reported to reduce the thermal flocculation of aqueous, clayey drilling fluids; polyacrylic acid having a molecular weight of from 4,000 to about 8,000 alone and with sulfonated polystyrene-maleic anhydride copolymer are reported as a high temperature dispersants for water based bentonite clay drilling fluids (see U.S. Pat. No. 4,476,029); and, a low molecular weight sulfonated copolymer of styrene and maleic anhydride (see U.S. Pat. No. 3,730,900). The last type has achieved considerable acceptance by the drilling mud industry as an effective and valuable product which is known as Mil-Temp.RTM. and is sold by Milchem, Inc. of Houston, Tex.
In recent years, Chesser et al have suggested in the J. Pet. Tech. 950-956 (1980) that poly(acrylic acid) is unsuitable as a high temperature rheological stabilizer of aqueous muds contaminated by calcium salts and/or containing high levels of clay solids.
It is an object of this invention to obtain a dispersant for water-based drilling fluids which remains operable even at high temperatures and in the presence of high levels of calcium ion contamination.
This and further objects will become apparent as the description of the invention proceeds.